In recent years, the demand to increase energy efficiency has risen sharply for all products. Efficiency factor has become a selling point, particularly for power supply units. The efficiency factor depends on many parameters, inter alia on the utilization of the power supply unit.
In particular, power supply units for computer systems are operated for long periods in what is called low-load operation. The efficiency curve of a power supply unit in low-load operation is thus particularly important. This power range is required, for example, in an energy-saving mode (standby) or some other operating mode with reduced power consumption of a computer system connected to the power supply unit.
Many methods are used to improve efficiency at low-load. For example, better components are used or the power supply unit is operated in the lower load range in what is known as burst mode.
In a classic power supply unit for computer systems, which can be designed, for example, as a switching-mode power supply, several voltage converters are used. Generally speaking, such a power supply unit comprises at least one primary or first converter circuit (main converter) and a second converter circuit (auxiliary power supply unit).
The main converter is designed for maximum output of a computer system and has an efficiency curve which describes a good efficiency, in particular in higher power ranges (for example, between 30 and 60 watts). In contrast, the auxiliary power supply unit is designed for lower outputs in a standby operation and has an efficiency curve which represents a favorable efficiency only for low outputs (up to about 30 watts).
To exploit the more favorable efficiency of the auxiliary power supply unit compared to the main power supply unit in standby operation and, conversely, in a higher load range to exploit the more favourable efficiency of the main power supply unit compared to the auxiliary power supply unit, conventional switching-mode power supply units with different converter circuits for different power ranges provide for the main converter to be switched on and off. Thus, in standby operation the auxiliary power supply unit is generally active, whereas in the higher power range for a main operation both the main converter and the auxiliary power supply unit are active.
To control the power of the converter circuits in a power supply unit, use is made of control circuits, which regulate the relevant converter circuit to a required power output based on power consumption of the computer system. In this connection, for example, a control circuit can be provided for each converter circuit in the power supply unit, i.e. a first control circuit for the first converter circuit (main converter) and a second control circuit for the second converter circuit (auxiliary power supply unit).
If it is desired to use the more favorable efficiency of the auxiliary power supply unit also for operating states and/or circuit parts of a connected device normally supplied by the main power supply unit, the outputs must be interconnected.
The drawback of such a solution is that during operation of the first converter circuit (main converter), the control circuit of the second converter circuit (auxiliary power supply unit) adjusts the output power of the latter downwardly and switches it off completely. If in such a case a switch-over is made from the first converter circuit to the second converter circuit, then the second converter circuit first has to be started up (for example, using what is known as a soft-start process). Because of the unavoidable duration of the start-up process of the second converter circuit, voltage drops may occur, with possibly problematic loss of supply for a computer system. This can lead to an error in the computer system and in the worst case to failure of electrical components and assemblies in the computer system.
Alternatively, it is possible to let the first converter circuit continue to run until the second converter circuit is operational or to keep both converter circuits permanently active in parallel. Both solutions have the disadvantage, however, that the control circuit of the converter circuit having the lower output voltage will switch off this converter circuit.
It could therefore be helpful to provide a combination of two interconnected converter circuits to increase efficiency in a power supply unit, while nevertheless preventing the risk of a voltage drop when switching over between the two converter circuits and minimizing losses in the power supply unit.